The present invention relates to the field of digital tuning systems.
A number of digital tuning systems for controlling a voltage controlled oscillator to generate a local oscillator signal for tuning a radio or television receiver are known. These digital tuning systems may be generally categorized as being either of the frequency synthesizer, voltage synthesizer or voltage sweep type.
Frequency synthesizers are typically closed loop. One type of frequency synthesizer includes a phase or frequency comparator for generating the control voltage for a local oscillator signal by comparing the phase and/or frequency of a variable frequency signal derived by the frequency division of the local oscillator signal and a relatively stable reference frequency signal. The frequency of the loop and thereby the frequency of the local oscillator signal is determined by division factors of fixed and programmable frequency dividers in the loop. The programmable divider is controlled in response to binary signals representing the number of a selected channel to determine the particular local oscillator frequency. Another type of frequency synthesizer includes a counter for counting in cycles of a voltage controlled local oscillator signal and a count comparator for comparing the number accumulated by the counter with a number derived from binary signals representing the channel number of a selected channel to develop a local oscillator control voltage. In either system channel numbers of selected channels can be readily displayed in response to the binary signals representing the number of the selected channels. Although such frequency synthesizers are advantageous in that the frequencies of the local oscillator signal are relatively accurate because of the closed loop nature of the systems, such systems are relatively expensive due to the cost of the high speed dividers and counters they necessarily employ.
Voltage synthesizers are typically open loop systems and generally include a memory having a plurality of tuning voltage memory locations for storing binary signals representing the tuning voltages for each of the channels that a user may select. The channel numbers of selected channels can be readily displayed, for example, in response to binary signals representing the channel numbers and utilized to address corresponding tuning voltage memory locations. Although such voltage synthesizers are advantageous in that they are relatively inexpensive compared with frequency synthesizers because they do not require high speed frequency dividers and counters, they tend to be less accurate because the required precision and resolution in converting the binary signals stored in the tuning voltage memory locations to the corresponding tuning voltages is not readily attainable in open loop systems.
Many tuning systems of the voltage sweep type are known. Bascially, they all generate a ramp-like tuning voltage which is utilized to sweep the frequency of the local oscillator signal. In its simplest form, the magnitude of the tuning voltage is increased or decreased under user control by means of a potentiometer or the like until the user determines that an acceptable station has been reached. Signal sweeping systems are also known in which the magnitude of a tuning voltage is changed until a carrier is automatically detected. Such sweep systems are advantageous in that they are relatively accurate compared to voltage synthesizers since the tuning voltage is continuously adjusted until an acceptable channel is located and are relatively inexpensive compared to frequency synthesizers since they do not require high speed frequency dividers and counters. However, since the tuning voltage is not derived as part of an operation involving the use of binary signals representing the number of a selected channel, additional apparatus must be provided for channel identification.
While it is possible to employ high speed counters to determine the frequency of the local oscillator signal and from the frequency derive the number of the selected channel, the use of high speed counters is to be avoided to maintain the cost effectiveness of sweep type systems.
Apparatus are also known for monitoring the channel to which a receiver is tuned by examining the tuning voltage. In these systems the tuning voltage for a selected channel to which the receiver is already tuned is compared with voltages having magnitudes corresponding to the magnitudes of the tuning voltage to tune respective channels stored in memory locations of a memory. The memory locations are successively addressed until there is at least an approximate equality between the tuning voltage and one of the stored voltages. The number of the selected channel is derived from the address of the memory location at which the approximate equality existed. Such systems may be used by television rating services to identify a limited number of channels in a particular viewing area. However, they are not particularly well suited for television receivers to identify all of the channels in the television tuning range because of the need for greater resolution in accurately distinguishing between closely spaced channels in the latter application. Moreover, such monitoring systems are not particularly well suited for sweep type systems to display the channel numbers of channels passed before an acceptable channel is located since the tuning voltage changes until an acceptable channel is located. In sweep systems, it may be desirable to display the channel number of channels passed to reach the acceptable channel so that users have a visible indication that the system is operating and are therefore not annoyed by apparent lack of operation as an acceptable channel is sought.